The invention relates to a process for the metal-removing machining of the end faces and also circumferential faces, which may be arranged both centrically and eccentrically, on crankshafts or similar workpieces.
Owing to the eccentrically positioned, for example peripheral, surfaces to be machined of the crankpin journals or end faces of the webs and the instability of the crankshaft, caused by its shape, as a workpiece, straight crankshafts always place high demands on the machining processes.
Crankshafts are usually rough-machined by metal-removing machining using specific cutting edges, that is to say by turning, milling, turn broaching, etc., and then the precision-machining to a dimension is carried out by means of grinding, hardening of the workpiece surfaces having hitherto generally been carried out subsequent to the metal-removing rough-machining and prior to the grinding.
Although in theory metal-removing machining processes are available which can be used to machine both the centric centre bearings and also the eccentric big-end journals and the ends of the web surfaces, for reasons of economy machining is generally performed using different processes:
Thus, in recent times rotary turn broaching has become established for machining the centre bearings, in which process work is carried out on the rapidly rotating workpiece, which is chucked on the centre-bearing axis, using an internal broaching tool which has been bent into a curved contour, generally a circular contour or a circular-segment contour.
The eccentric end faces of the webs are generally also machined at the same time.
By contrast, for machining the eccentric crankpin journals, that is to say the peripheral surfaces thereof, internal milling, that is to say milling with an annular milling cutter which has inwardly directed cutting edges on its inner circumference, has become the established procedure. In this process, the crankshaft generally does not rotate during the machining, but rather the internal-milling cutter rotates and additionally moves on a circular path around the crankpin journal situated in the free interior of the internal-milling cutter. The cutting speed is in this case achieved exclusively by the movement of the internal-milling cutter. One of the drawbacks of this process is that particularly in the machining of journals, owing to the identical directions of curvature of journal and milling cutter, the result is a considerable wrap, that is to say that frequently a plurality of cutting edges are in action at the same time. This results in the introduction of high transverse forces into the workpiece. Due to the crankshaft being stationary, it is possible simultaneously to machine a plurality of crankpin journals by means of respectively separate internal-milling units, as long as the corresponding crankpin journals are also spaced sufficiently far apart in the axial direction to allow different internal-milling units to act on them.
The processes described have become established in the mass production of crankshafts, for example for engines for passenger cars, due to the fact that they exhibit sufficient machining accuracy while also permitting very much lower machining times per crankshaft or per journal than, for example, turning.
Although the maintenance, repair, refitting and adjustment of throw-away cutting-tool tips, etc. is more difficult in the case of such an internally toothed milling cutter, such a tool in principle offers the advantage of good and stable mounting in a tool support which surrounds the annular tool on all sides. This is necessary since, owing to the crankshaft being stationary, the support has to be able to move the annular tool during internal milling in a defined manner not only in the X-direction but also in the Y-direction. However, the rotational speed of the internal-milling cutter generally remains constant during a revolution around the bearing journal to be machined and is independent of the current relative position of workpiece and tool with respect to one another.
It is sufficient for the support which rotates the internal-milling cutter to be displaceable exclusively in the X-direction but not also in the Y-direction only if, on the one hand, the workpiece is driven in rotation in a defined manner during the machining (C-axis of the workpiece) and, on the other hand, the free internal diameter of the internal-milling cutter is greater than the throw of the largest crankshaft to be machined.
Particularly in the case of machines in which a plurality of tool supports are present, in order to work with a plurality of tools, that is to say, for example, internal-milling cutters, simultaneously on the same workpiece, all things considered it represents a simplification and saving if it is possible for each of the plurality of supports to do without the Y-axis, even though it is then additionally necessary for the C-axis to be present for the rotation which is then necessary at the spindle heads for the crankshaft, which as a rule is chucked and driven on both sides.
In this case, when simultaneously machining a plurality of crankpin journals, the crankshaft rotates at a constant speed and the various internal-milling cutters also rotate at the same speed, which remains constant during a journal-machining operation.
A milling tool of this kind is [lacuna] with a multiplicity of cutting edges, which are generally arranged as throw-away cutting-tool tips on an annular body (in the case of an internal-milling cutter) or a disc-like body (in the case of an external-milling cutter), in the internal circumferential region or in the external circumferential region. The reconditioning or refitting or resetting of such milling cutters firstly takes considerable time and, owing to the not inconsiderable costs of the cutting means used, is expensive.
The machining costs per crankshaft, which are to be minimized, therefore proportionately include not only the investment costs for the machine, the share of which decreases per crankshaft as the machining time falls, but also a cost factor which is affected by the life of the tool and should be minimized.